This invention relates to antennas, and more particularly to an antenna based on a tapered helix configuration and having low VSWR over a wide bandwidth, and multi-mode operation.
Most antennas are capable of efficient operation over only a limited range of frequencies. For efficient energy radiation or reception, conventional antenna designs calls for antenna dimensions that are on the same order as the operating wavelength (xcex). Efficient antenna operation requires not only efficient radiation or reception, the antenna must also be matched to the specific source or load for maximum energy transfer.
Antenna match quality is determined by the voltage standing wave ratio (VSWR) of the antenna at each frequency of interest. A perfect match requires a one-to-one ratio at all frequencies.
For broadband applications, there are several types of antennas that can be designed to provide a low VSWR over wide frequency ranges. Some of these antennas are inherently directional such as the conical spiral and log periodic. Others are omni-directional such as the bicone and tapered blade antennas.
Although there are a number of antenna configurations that can provide a low VSWR over a wide band, most have some limitations that make them unacceptable for many applications. For example, a log periodic antenna can easily be designed to provide low VSWR over several frequency octaves. But, the log periodic phase center, the effective radiating point of the antenna, varies with frequency and a log periodic is physically quite large compared to the wavelength of its lowest operating frequency. A bicone antenna or its monopole is capable of providing low VSWR over a wide bandwidth, but occupies a large volume compared to narrow band antennas having the same low end operating frequency. Generally, wideband antennas are difficult to design for low VSWR over more than an octave frequency range and are generally much larger than a wavelength at their lowest usable frequency.
Antennas, whether broadband or not, are not required to have low VSWR; many conventional antenna designs typically exhibit a high VSWR ( greater than 3:1). However, a high VSWR adversely affects efficiency, unless some form of compensatory matching network is used. But matching networks create new problemsxe2x80x94most matching networks are not broadband and they tend to decrease the power available for transmission. For high power transmitters, matching networks must often be designed with electro-mechanical tuning elements. Such designs are costly and make automation of the matching function must slower than is possible with lower-power solid state tuning elements. In general, impedance matching to achieve a low VSWR is relatively easy for narrow band antennas ( less than 10% of center frequency), but more difficult for wideband antennas ( greater than 20% of center frequency).
One aspect of the invention is a wideband multi-mode antenna. In its simplest form, the antenna is made from a single right triangularly shaped sheet of conductive material, having a height and a base dimension. The planar material is rolled, such that the antenna has the height of the planar material, a number of turns having spacing between them, a base diameter, and a pointed tip.
An advantage of the invention is that the antenna is compact, low cost, and easily manufactured. It provides a low VSWR that can be easily matched over a wide range of frequencies. With these features, the antenna has ready application to many existing systems as well as to new systems now under development for the wireless market. The potential for small size and wide bandwidth make the antenna especially useful for mobile communications and multi-mode radios.